Conventionally the pistons employed in axial piston pumps and motors are hollow and contain a lightweight filler material to reduce the high inertia forces acting on the piston during operation of the pumps and motors. A passage normally extends through the filler material and through the base portion of the piston to transmit hydraulic fluid through the piston to lubricate the bearing surfaces of the components associated with the piston. Examples of such filled pistons are described in U.S. Pat. No. 3,984,904 which issued to Charles H. Schlecht on Oct. 12, 1976, U.S. Pat. No. 3,707,113 which issued to Hein et al on Dec. 26, 1972 and U.S. Pat. No. 3,633,467 which issued to Watanabe et al on Jan. 11, 1972.
One of the problems encountered with such filled pistons is the formation of cracks or cavities in the passage in the filler material during the manufacturing process. It has been found that the high pressure hydraulic fluid passing through the passage tends to erode the crack or cavity. Such eroded material is then carried into the bearing surfaces associated with the piston thereby damaging the bearing surfaces. In some cases, the eroded material blocks the passage in the base portion of the piston and thereby blocks the flow of lubricating fluid to the bearing surfaces which again causes damage to the bearing surfaces.
Because of the above problem, it is necessary to visually inspect the passage in every piston with a cystoscope type instrument to insure that there are no cracks or cavities in the passage. Such 100% inspection adds to the manufacturing time and thus increases the total cost of manufacturing the piston. Moreover because of the stringent requirements that the fluid passage not have any visible cracks therein, approximately 40% of the filled pistons are scrapped.
The present invention is directed to overcoming one or more of the problems as set forth above.